You may find HERE the very interesting results of one of the most extensive study of religions in the US.
This is how it is presented on the web site:
The 2010 U.S. Religion Census: Religious Congregations & Membership Study (RCMS) is a county-by-county enumeration of religious bodies in the US. A quick overview is available here. It is an update of the 1952, 1971, 1980, 1990, and 2000 studies originally done by the National Council of Churches and the Glenmary Research Center. Since 1990, the Association of Statisticians of American Religious Bodies (ASARB) has sponsored the studies.
Each participating religious body supplies the number of churches, full members, adherents, and attendees for each county. See “Data Collected” for more information. Our data collection office can assist any group with compiling their data to the county level.
You may download HERE the summary of the findings.